In advanced optical systems presently used and under development it is frequently required that an optical beam such as a laser beam, for example, be accurately and very precisely directed in angular disposition with respect to a relatively small distant target. Since the size of the beam is customarily very small, extremely high accuracy in directing or steering the beam is required to prevent the loss or attenuation of light energy transmitted by the beam which would thereby detract from the effectiveness and efficiency of the optical system in which it is used.
A closely analogous problem is tracking a light energy source, since in both cases a reflective element must frequently be angularly positioned with an accuracy of fractions of a microradian.
In the present state of the art there are three principal methods and techniques which are employed for accomplishing high precision beam steering and tracking. One such prior art method involves the employment of two orthogonally oriented galvanometers which are energized to control the movement of a reflective surface relative to two orthogonally related axes of movement.
One major problem with the galvanometer type of system and technique is the requirement for having to locate two galvanometer driven reflective members which is frequently inconvenient by reason of space considerations. Moreover, the distance between the two galvanometer units often presents a problem due to beam divergence and different distances to the target. Additionally, the galvanometer technique and method of beam steering inherently involves a problem of hysteresis, i.e., the reflective surface does not completely return to its precise initial position after having undergone actuation in response to energization of the galvanometer movements. Since there are two galvanometer movements involved, each of which positions a reflective surface relative to a different orthogonally related axes of movement, such hysteresis problems can be compounded and thereby lead to serious difficulty.
Another method which employs two orthogonally oriented servo driven mirrors also requires two separate reflective units and therefore necessarily similarly involves the problem of space considerations and the location of the two separate units with respect to each other; undesirably the distance between the units can give rise to unwanted beam divergence and also different distances to the target. Furthermore, servomechanism drives are generally relatively large and bulky, particularly where precision beam steering involves a highly precise optical beam such as a laser beam, for example, and the driven reflective surface may be of the order of only several centimeters in diameter.
A third technique employs the use of flexure responsive piezo-electric elements as motor transducers; i.e., they convert on electrical input to a mechanical output so as to change the angular disposition of a baseplate to which they're attached. These devices, which are known under the trademark BIMORPH, are essentially laminates of piezoelectric ceramic material in a disk form supporting the reflective surface. Two sets of electrodes orthogonally oriented on the disk are connected to a suitable source of electrical control signals. Upon energization by the control signals, the piezoelectric material is caused to bend (somewhat in the manner of a bimetallic spring upon the application of heat) thus angularly changing the disposition of the baseplate to which they are attached and the reflective surface supported thereon.
One deficiency of the piezoelectric disk technique, however, is that only relatively extremely small deflection angles are obtainable within the limits of the extreme deflection realized upon the application of appropriate electrical control signals. Additionally, relatively high voltages are required to actuate the piezoelectric types of devices which, though not a prime consideration, may be a disadvantage in certain types of systems.
The major problem, however, with the piezoelectric laminate technique is that an extremely high degree of hysteresis is encountered in their use so that a corrective signal input is often required to overcome the amount of hysteresis which prevents the reflective surface from being returned to precisely its original starting angular position.
These prior art devices, though operationally feasible, present numerous disadvantages which, if obviated, could significantly improve beam steering systems which angularly position a reflective surface so as to point a laser beam or track a light energy source, for example.
Accordingly, it is highly desirable that a beam steering system be devised which eliminates many, if not all, of the disadvantages of the known prior art systems and techniques used to point a laser beam or track a light energy source.